CVE-2021-43890
Microsoft Windows AppX Installer Spoofing Vulnerability
Does this matter?
Known to be exploited in the wild (CISA KEV, CISA remediation deadline 29 December 2021). Treat as an emergency change: patch or isolate now, then hunt for prior compromise.
Description
We have investigated reports of a spoofing vulnerability in AppX installer that affects Microsoft Windows. Microsoft is aware of attacks that attempt to exploit this vulnerability by using specially crafted packages that include the malware family known as Emotet/Trickbot/Bazaloader. An attacker could craft a malicious attachment to be used in phishing campaigns. The attacker would then have to convince the user to open the specially crafted attachment. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights. Please see the Security Updates table for the link to the updated app. Alternatively you can download and install the Installer using the links provided in the FAQ section. Please see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability. December 27 2023 Update: In recent months, Microsoft Threat Intelligence has seen an increase in activity from threat actors leveraging social engineering and phishing techniques to target Windows OS users and utilizing the ms-appinstaller URI scheme. To address this increase in activity, we have updated the App Installer to disable the ms-appinstaller protocol by default and recommend other potential mitigations.
- CVSS 3.1
- 7.1 HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
- EPSS
- 11.29% probability · 96th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Listed 15 December 2021 · due 29 December 2021 · used in ransomware campaigns
- Affected
- microsoft/app installer
- Source
- secure@microsoft.com
EPSS 11.3% since 30 October 2026; no change recorded yet. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
CISA notes
Apply updates per vendor instructions. https://nvd.nist.gov/vuln/detail/CVE-2021-43890
References
- https://github.com/ChrisTitusTech/winutil/pull/26Issue Tracking
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890Patch, Vendor Advisory
- https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.htmlPress/Media Coverage, Third Party Advisory
- https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/Press/Media Coverage, Third Party Advisory
- https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/Exploit, Vendor Advisory
- https://github.com/ChrisTitusTech/winutil/pull/26Issue Tracking
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890Patch, Vendor Advisory
- https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.htmlPress/Media Coverage, Third Party Advisory
- https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/Press/Media Coverage, Third Party Advisory
- https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/Exploit, Vendor Advisory
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-43890US Government Resource
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.